PHY_10_3_V2_P_Measuring magnetic flux density

1

To explain the physical meaning of magnetic induction vector based on problem solving and modern technological advances (magnetic levitation train, etc.)

2

What is the physical meaning of magnetic flux density?

The magnetic flux density at a point in space is the force experienced per unit length by a long straight conductor carrying unit current and placed at right angles to the field at that point.

The unit for magnetic flux density is the tesla, T. It follows
from the equation for B that 1 T = 1 N A−1 m−1.
The tesla is defined as follows:

The magnetic flux density is 1 T when a wire carrying a
current of 1 A placed at right angles to the magnetic field
experiences a force of 1 N per metre of its length.

1. Measuring B with a Hall probe
The simplest device for measuring magnetic flux density B is a Hall probe (Figure 4.1).

Figure 4.1

Figure 4.2.

2. Measuring B with a current balance
Figure 4.3 shows a simple arrangement that can be used to determine the flux density between two magnets.

Figure 4.3

Figure 4.4.

A rectangular loop of wire hangs vertically as shown in Fig. A magnetic field is directed horizontally, perpendicular to the plane of the loop, and points out of the page as represented by the symbol The magnetic field is very nearly uniform along the horizontal portion of wire ab (length l=10.0 cm) which is near the center of the gap of a large magnet producing the field. The top portion of the wire loop is out of the field. The loop hangs from a balance (reads 0 when B=0) which measures a downward magnetic force of F = 3.48 × 10-2 N when the wire carries a current I = 0.245 A.
What is the magnitude of the magnetic field B?

APPROACH Three straight sections of the wire loop are in the magnetic field: a horizontal section and two vertical sections. We apply Eq. 20–1 to each section and use the right-hand rule.
SOLUTION Using right-hand-rule-2, we see that the magnetic force on the left vertical section of wire points to the left, and the force on the vertical section on the right points to the right. These two forces are equal and in opposite directions and so add up to zero. Hence, the net magnetic force on the loop is that on the horizontal section ab, whose length is l=0.100 m. the angle θ between and the wire is θ=900, so sin θ=1.

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